The present invention relates to a radiation detection circuit for an X-ray CT scanner and a data acquisition system for acquiring detection results from a plurality of such radiation detection circuits.
In an X-ray CT scanner, a fan X-ray beam radiated from an X-ray tube and shaped by a collimator passes through an object set in a photographing area and is detected by a plurality of X-ray detectors arranged to face the X-ray tube.
Output currents of the respective detectors are integrated as charge information during a short period of measurement. The measurement is repeated while the positions of the X-ray tube and the detector array are rotated around the object a small angle at a time. Parallel data of the respective measurements is projection data of slices of the object at the respective angles. A plurality of projection data are sequentially obtained. Such parallel data are collected by the data acquisition system for each measurement, and the collected data are sequentially digitized by an A/D converter, analyzed by a computer and image-reconstructed to produce an X-ray tomogram of the object.
JP-A-61-263441 shows a prior art data acquisition system. In this system, as shown in FIG. 9, a current output from an X-ray detector (Xe chamber) is utilized without loss so that the current output from the X-ray detector is integrated as charge during a period of measurement, which is then converted to a voltage.
In FIG. 9, numeral 7-i denotes an integrator which is made up of switches S1i, S2i and S3i and an integration capacitor Csi. As many such integrators as the number of X-ray detectors are provided. Numeral 8 denotes a charge/voltage converter which is made up of an operational amplifier OP1, switches SW4 and SW7, a hold capacitor C.sub.H and an offset compensation circuit 9. In the charge/voltage converter, the switches S1i and S3i are opened and the switch S2i is closed to integrate the current output of the detector in the integration capacitor Csi during the measurement period. Then, the switch S1i is closed and the switch S2i is opened. Then, the switches S3i of the respective integration circuits are sequentially closed and opened to transfer the charges of the integration capacitors Csi to the hold capacitor C.sub.H to convert the charge to a voltage.
In the prior art, where the current output of the X-ray detector is I.sub.i and one measurement period is t, an output voltage V generated in the integration capacitor Csi assumes a maximum value V.sub.max when I.sub.i is maximum EQU V.sub.max =(I.sub.i .multidot.t)/C (1)
where C is a capacitance of the integration capacitor Csi.